Modern wind turbine blades are typically manufactured using Vacuum Assisted Resin Transfer Moulding, a process that produces strong, lightweight composites by infusing resin into compacted reinforcing materials under vacuum. A large part of reinforcing material is usually glass or carbon fibre woven mats.
For deeply concave shapes of the inner face of the mould there may be the risk that the fibre mats are not maintained in a position firmly against the mould during lay-up. Instead of following the actual curvature of the mould the fibre mats may in certain circumstances tend to take on the shape of catenaries (like a hanging chain), leaving voids between the inner surface of the mould and the fibre mats (“hovering glass”). If several layers of fibre mats are placed on top of each other, friction between the layers may be strong enough to prevent the fibre mats from being pressed against the mould when vacuum is applied. In the subsequent moulding process the voids between the surface of the mould and the fibre mats will be filled with resin that is not reinforced by any fibre material. As a result, the structural characteristics of the blade in the regions of “hovering glass” may not be as desired.
In addition, if on application of vacuum the glass is pressed partly or completely out into the void this may result in wrinkles and folds of the fibre mats, which may in turn lead to mechanical weaknesses if the wrinkles and folds are not flattened before the resin is injected.
In US 2003/0077965 A1 a three-dimensional spacer fabric resin infusion medium and reinforcing composite lamina for use in the manufacture of fibre reinforced polymer composites is disclosed. The use of the three-dimensional spacer fabric as a composite lamina aids in both the resin infusion rate and resin uniformity throughout the laminate.
In WO 2007/038930 A1 a Resin Transfer Moulding method of producing a fibre reinforced product is disclosed. The method comprises the steps of: a) placing at least one porous member in a mould; b) placing one or more layer(s) of reinforcing fibres in the mould; c) introducing resin for distribution through the porous member to the fibre layers; and d) allowing the resin to cure and the distribution member to coalesce to form a continuous layer.
In EP 1 310 351 B1 a method for making a windmill blade of composite materials which include a fibre reinforced matrix material is disclosed, wherein the blade is made in one piece in a closed mould. The method comprises the following steps: A mould core with a flexible external core part and an internal, firm or workable core part, and outer mould parts are arranged to close around the mould core for formation of a mould cavity there between. Composite material and possible core inserts are laid on an outer mould part and/or the mould core. The outer mould parts are closed around the mould core and around the composite material placed in the mould cavity. The composite material is set. The outer mould parts are removed. The mould core is taken out of the shape permanent blade before or after removing the outer mould parts. Some of the required matrix material is used in connection with the reinforcing fibre when laying the composite material and where additional matrix material is added after closing the mould. The method involves use of composite materials such as glass or carbon fibre reinforced epoxy, polyester, vinyl ester, or thermoplastic.